1. Field of the Invention
The present invention relates to an ignition coil for a vehicle and, particularly, a stick-type resin-insulated ignition coil to be mounted in each plug hole of a vehicle engine.
2. Description of the Related Art
JP-A-2000-182859 discloses a stick-type ignition coil that includes a coil housing and a tower housing. The coil housing accommodates a center core, a primary coil and a secondary coil, and the tower housing accommodates a high-tension terminal. Insulation epoxy resin is filled in the coil housing and the tower housing to insulate the accommodated members from each other.
As shown in FIG. 8, a high tension terminal 101 of a known ignition coil 100 is a bullet-like metal member. The tower housing 104 is a cylindrical member made of resin. The high tension terminal 101 is disposed in the tower housing 104 to be coaxial therewith. The high tension terminal 101 has an upper flange 102 and a middle barrel 103. There is a comparatively small circumferential gap 105 between the inside surface of the tower housing 104 and the outer periphery of the upper flange 102. The high tension terminal 101 is held by a cylindrical guide 79 to be coaxial with the tower housing 104, so that a projection 75 can be inserted into the lower end of a spool easily when assembled. There is a comparatively large circumferential gap 106 between the outer periphery of the middle barrel 103 and the inside surface of the tower housing 104. Although the circumferential gap 105 around the high tension terminal 101 is comparatively small, liquid state epoxy resin is poured through the circumferential gap 105 into the space formed between the middle barrel and the tower housing 104. Thereafter, the epoxy resin is solidified in the smaller circumferential gap 105 and the larger circumferential gap 106.
Because the high tension terminal 101 is made of metal while the tower housing 104 is made of resin, the coefficient of thermal expansion of the high tension terminal 101 is much different from that of the tower housing 104. The temperature of the ignition coil 100 increases when it is operated and decreases after it is stopped. Therefore, the high tension terminal 101 and the tower housing 104 are heated and cooled cyclically. Accordingly, the epoxy resin disposed in the circumferential gaps 105, 106 is subjected to thermal stress.
As shown in FIG. 8, the high tension terminal 101 has the middle barrel 103 that has a smaller outside diameter than the upper flange in order to elongate the mean distance between the high tension terminal 101 and the inside surface of a plug hole (indicated by 91 in FIG. 7) formed in an engine. This structure reduces stray capacitance between the high tension terminal and the plug hole to increase the high tension voltage. However, voids 107 may be formed in the epoxy resin disposed in the gap 106 when liquid state epoxy resin is poured into the large gap 106 through the small gap 105. If there are voids 107 in the epoxy resin disposed between the high tension terminal 101 and the tower housing 104, insulation performance of the ignition coil lowers, and the ignition coil may not supply sufficient high tension voltage to a spark plug.
Therefore, an object of the present invention is to provide a reliable ignition coil in which voids are not formed in the insulation resin disposed between the tower housing and the high tension terminal.
According to a feature of the invention, an ignition coil includes a center core with primary and secondary coils wound around, a cylindrical coil housing, a cylindrical tower housing having an upper flange and a middle barrel disposed under the coil housing, a high tension terminal connected to the secondary coil and solid insulation resinous members that are solidified after being filled in the coil housing and the tower housing. In the above ignition coil, at least the tower housing or the upper flange has a void-purging passage for purging air from the insulation resinous members when filled in the coil housing and the tower housing.
When liquid state insulation resin is poured from the upper end of the coil housing into- the inside of the coil housing and the tower housing, air can be purged from the circumferential gap and the void-purging passage.
Preferably the void-purging passage is vertically formed in the upper flange. The void-purging passage may be a groove disposed on the inside surface of the tower housing. The high tension terminal may be molded in the tower housing so as to eliminate a chance to form voids around the high tension terminal.